fix: use UI flow context for sign_up vs login telemetry (#10388)

## Problem

GA4 shows **zero** `sign_up` events from cloud.comfy.org. All new users
(Google/GitHub) are tracked as `login` instead of `sign_up`.

**Root cause:** `getAdditionalUserInfo(result)?.isNewUser` from Firebase
is unreliable for popup auth flows — it compares `creationDate` vs
`lastSignInDate` timestamps, which can differ even for genuinely new
users. When it returns `null` or `false`, the code defaults to pushing a
`login` event instead of `sign_up`.

**Evidence:** GA4 Exploration filtered to `sign_up` + `cloud.comfy.org`
shows 0 events, while `login` shows 8,804 Google users, 519 email, 193
GitHub — all new users are being misclassified as logins.

(Additionally, the ~300 `sign_up` events visible in GA4 are actually
from `blog.comfy.org` — Substack newsletter subscriptions — not from the
app at all.)

## Fix

Use the UI flow context to determine `is_new_user` instead of Firebase's
unreliable API:
- `CloudSignupView.vue` → passes `{ isNewUser: true }` (user is on the
sign-up page)
- `CloudLoginView.vue` → no flag needed, defaults to `false` (user is on
the login page)
- `SignInContent.vue` → passes `{ isNewUser: !isSignIn.value }` (dialog
toggles between sign-in/sign-up)
- Removes the unused `getAdditionalUserInfo` import

## Changes

- `firebaseAuthStore.ts`: `loginWithGoogle`/`loginWithGithub` accept
optional `{ isNewUser }` parameter instead of calling
`getAdditionalUserInfo`
- `useFirebaseAuthActions.ts`: passes the option through
- `CloudSignupView.vue`: passes `{ isNewUser: true }`
- `SignInContent.vue`: passes `{ isNewUser: !isSignIn.value }`

## Testing

- All 32 `firebaseAuthStore.test.ts` tests pass
- All 19 `GtmTelemetryProvider.test.ts` tests pass
- Typecheck passes

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-10388-fix-use-UI-flow-context-for-sign_up-vs-login-telemetry-32b6d73d3650811e96cec281108abbf3)
by [Unito](https://www.unito.io)

---------

Co-authored-by: GitHub Action <action@github.com>

src/components/dialog/content/SignInContent.vue

@@ -186,13 +186,13 @@ const toggleState = () => {
 }
 
 const signInWithGoogle = async () => {
-  if (await authActions.signInWithGoogle()) {
+  if (await authActions.signInWithGoogle({ isNewUser: !isSignIn.value })) {
     onSuccess()
   }
 }
 
 const signInWithGithub = async () => {
-  if (await authActions.signInWithGithub()) {
+  if (await authActions.signInWithGithub({ isNewUser: !isSignIn.value })) {
     onSuccess()
   }
 }